Re: [PATCH v2 3/3] trace: Example of "centralized" recorder tracing

[Daniel P . Berrangé]

On Fri, Jun 26, 2020 at 06:27:06PM +0200, Christophe de Dinechin wrote:
> This is an example showing how the recorder can be used to have one
> "topic" covering multiple entries. Here, the topic is "lock".
> 
> Here are a few use cases:
> 
> - Checking locks:
>     RECORDER_TRACES=lock qemu
> - Graphic visualization of locks:
>     RECORDER_TRACES="lock=state,id" qemu &
>     recorder_scope state
>     <Hit the 't' key to toggle timing display>
>     <Hit the 'c' key to dump the screen data as CSV>
>     cat recorder_scope_data-1.csv
> 
> Signed-off-by: Christophe de Dinechin <dinechin@redhat.com>
> ---
>  util/qemu-thread-common.h | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/util/qemu-thread-common.h b/util/qemu-thread-common.h
> index 2af6b12085..0de07a471f 100644
> --- a/util/qemu-thread-common.h
> +++ b/util/qemu-thread-common.h
> @@ -15,6 +15,9 @@
>  
>  #include "qemu/thread.h"
>  #include "trace.h"
> +#include "trace/recorder.h"
> +
> +RECORDER_DEFINE(lock, 16, "Lock state");
>  
>  static inline void qemu_mutex_post_init(QemuMutex *mutex)
>  {
> @@ -23,12 +26,14 @@ static inline void qemu_mutex_post_init(QemuMutex *mutex)
>      mutex->line = 0;
>  #endif
>      mutex->initialized = true;
> +    record(lock, "Init state %d for %p", -1, mutex);
>  }
>  
>  static inline void qemu_mutex_pre_lock(QemuMutex *mutex,
>                                         const char *file, int line)
>  {
>      trace_qemu_mutex_lock(mutex, file, line);
> +    record(lock, "Locking state %d for %p", 1, mutex);
>  }
>  
>  static inline void qemu_mutex_post_lock(QemuMutex *mutex,
> @@ -39,6 +44,7 @@ static inline void qemu_mutex_post_lock(QemuMutex *mutex,
>      mutex->line = line;
>  #endif
>      trace_qemu_mutex_locked(mutex, file, line);
> +    record(lock, "Locked state %d for %p", 2, mutex);
>  }
>  
>  static inline void qemu_mutex_pre_unlock(QemuMutex *mutex,
> @@ -49,6 +55,7 @@ static inline void qemu_mutex_pre_unlock(QemuMutex *mutex,
>      mutex->line = 0;
>  #endif
>      trace_qemu_mutex_unlock(mutex, file, line);
> +    record(lock, "Unkocked state %d for %p", 0, mutex);
>  }

IMHO the whole point of having the pluggable trace backend impls, is
precisely that we don't have to add multiple different calls in the
code. A single trace_qemu_mutex_unlock() is supposed to work with
any backend.

Regards,
Daniel
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